Method for preparing aluminum matrix composite using no pressure infiltration

ABSTRACT

The present invention provides a method for preparing an aluminum matrix composite by infiltrating aluminum into a preform within a short period of time without a pressurization configuration using a special device as compared to the existing pressure infiltration method. According to one aspect of the present invention, provided is a method for preparing an aluminum matrix composite using pressureless infiltration, the method including: preparing a preform formed of a mixture of raw powders capable of forming ceramic through a combustion synthesis reaction; immersing the preform in an aluminum melt, in which a part of the preform is exposed to an external environment without being immersed in the aluminum melt; and infiltrating molten aluminum into the preform while causing a combustion synthesis reaction in the preform.

TECHNICAL FIELD

The present invention relates to a method for preparing an aluminummatrix composite by distributing a non-metallic material, such asceramic, as a reinforcing phase (or a reinforcing material) on analuminum matrix to enhance mechanical properties, and more particularly,to a method for preparing an aluminum matrix composite usingpressureless infiltration.

BACKGROUND ART

An aluminum matrix composite is a material in which a non-metallicmaterial such as ceramic is distributed as a reinforcing phase in amatrix formed of pure aluminum or an aluminum alloy, is light-weight andhas high strength and rigidity and excellent resistance to wear andhigh-temperature properties, and thus has potential to be used as astructural material for transportation, and machinery and electricdevices and so on.

Mechanical properties of a metal matrix composite are greatly affectedby the type, size, shape, and volume fraction of a reinforcement,characteristics of interface between matrix and reinforcement. When acomposite is prepared by ex-situ method (introducing ceramicreinforcements into a molten metal), it was not easy to introduce theceramic reinforcements into a molten metal due to low wettabilitybetween the ceramic reinforcements and the matrix molten metal.

In order to solve these problems, a pressure infiltration process hasbeen developed, and the process is a method in which a preform is firstprepared using a reinforcing material powder, aluminum (described asonly aluminum for convenience, and hereinafter, aluminum will refer toaluminum and an aluminum alloy) molten metal is injected thereinto, andthen the preform is filled with the aluminum molten metal by highpressure (using a mechanical or gas pressure, and the like). The methodis advantageous in that the composite may be prepared within a shorttime, but has a problem in that a large complex apparatus is requiredfor applying high pressure.

In order to improve demerits of the pressure infiltration process,pressureless infiltration processes have been developed, and among theprocesses, a direct melt oxidation (DIMOX) process developed by LanxideCorp., is a representative process. The process is a method of preparinga composite of metal/ceramic by inducing an oxidation reaction at aninterface between a molten metal and a preform to simultaneously produceand grow an oxidation product (Urquhart, Mat. Sci. Eng. A144, 1991,75-82). However, the process is disadvantageous in that the molten metaltemperature is as high as 1,200° C. and the infiltration time is as longas 24 hours.

DISCLOSURE Technical Problem

The present invention has been made in an effort to solve variousproblems including the aforementioned problems and provides a method forpreparing an aluminum matrix composite by infiltrating aluminum into apreform within a short period of time without a pressurizationconfiguration using a special device as compared to the existingpressure infiltration method. However, this problem is illustrativeonly, but the scope of the present invention is not limited thereby.

Technical Solution

According to one aspect of the present invention, provided is a methodfor preparing an aluminum matrix composite using pressurelessinfiltration, the method including: preparing a preform of a raw powdersmixture capable of forming ceramic reinforcements through a combustionsynthesis reaction; immersing the preform in an aluminum molten metal,in which a part of the preform is exposed to an external environmentwithout being infiltrated into the aluminum molten metal; andinfiltrating molten aluminum into the preform while causing a combustionsynthesis reaction in the preform.

The infiltrating of the molten aluminum may further include allowing thepreform to be ignited.

Further, the method may further include immersing the entire preform inthe aluminum molten metal after the infiltrating of the molten aluminum.

The raw powder mixtures may be any one of a mixture of Ti and B₄Cpowders, a mixture of Al, B₂O₃, and C, a mixture of Al, B₂O₃, and TiO₂,a mixture of Ti and C, a mixture of Al, TiO₂, and C, a mixture of Al,Ti, and B₂O₃, and a mixture of Al, TiO₂, and B₄C.

The raw powder mixtures may further include an aluminum powder in anexcessive amount in addition to a stoichiometric amount required for acombustion synthesis reaction among the raw powders constituting themixture. In this case, the mixture of raw powders may further includeactivating powders which can promote the exothermic reactions, and theactivating powders may include one or more of a copper oxide, amanganese oxide, an iron oxide, and a nickel oxide.

In order to induce a partial combustion synthesis reaction, anon-metallic raw powder among the raw powders constituting the mixturemay be added in the mixture of raw powders in an excessive amount equalto or more than a stoichiometric amount required for the combustionsynthesis reaction.

Alternatively, the mixture of raw powders may further include a compoundpowder which does not participate in the combustion synthesis reaction,and the compound may include one or more of B₄C, SiC, TiC, and Al₂O₃.

Meanwhile, the temperature of the aluminum melt may be in a range of750° C. to 950° C.

Advantageous Effects

According to the present invention configured as described above, analuminum matrix composite may be prepared by using a combustionsynthesis reaction of a preform to be infiltrated by aluminum melt intothe preform within a short period of time without a special device ascompared to the existing pressure infiltration method. Accordingly, thepresent invention is economically efficient in terms of device and costas compared to the existing pressure infiltration process. In addition,since the process is completed at a low aluminum melt temperature in theatmosphere within a short time of a several minutes, the process isadvantageous in that the process time may be significantly reduced ascompared to the existing pressureless infiltration process whichrequires a long period of time, and the process temperature may also bereduced. A metal matrix composite prepared by the process islight-weight and excellent in mechanical properties such as elasticmodulus and hardness, and excellent in thermal stability, and thus, maybe utilized in parts which require high hardness, high stiffness andthermal stability. The effects of the present invention are not limitedto those mentioned above, and other effects which are not mentioned maybe clearly understood by a person with ordinary skill in the art towhich the present invention pertains from the following description.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a method for preparing an aluminum matrixcomposite according to the present invention.

FIG. 2 is a schematic view illustrating an infiltration process in apreform.

FIG. 3 is a graph illustrating a change in temperature of the preform inan aluminum molten metal.

FIG. 4 is a graph in which a relationship between the infiltrationlength-infiltration time is theoretically calculated.

FIGS. 5 to 8 are results of microstructures for aluminum matrixcomposites prepared according to experimental examples of the presentinvention.

FIG. 9 is a perspective view of a ring-type holder for fixing thepreform.

FIG. 10 illustrates immersing the preform in the aluminum molten metalusing the ring-type holder.

BEST MODE

Hereinafter, embodiments of the present invention will be described indetail as follows with reference to the accompanying drawings. However,the present invention is not limited to the embodiments to be disclosedbelow, but may be implemented in various forms different from eachother, and the following embodiments are provided to make the disclosureof the present invention complete and to completely inform a person withordinary skill in the art of the scope of the present invention.Further, in the drawings, sizes of constituent elements may beexaggerated or reduced for convenience of explanation.

FIG. 1 illustrates a schematic view of a method for preparing analuminum matrix composite according to the present invention. Referringto FIG. 1, provided is a preform 100 prepared by using a mixture of rawpowders capable of causing a combustion synthesis reaction (or alsoreferred to as a self-propagating high temperature synthesis). As aproduct of the combustion synthesis reaction, a hard ceramic materialsuch as carbide, oxide, and boride may be formed. When the combustionsynthesis reaction is used to produce a hard ceramic material as areinforcing phase in an aluminum matrix, the interfacial bondingstrength of the matrix/reinforcing phase is excellent because theceramic material is thermodynamically stable and the interface of thereinforcing phase is clean. For this reason, mechanical properties of ametal matrix composite prepared by using a combustion synthesis reactionare better than those of a composite prepared by the ex-situ method.

The preform 100 is prepared using a mixture of raw powders capable offorming a hard ceramic reinforcement through a combustion synthesisreaction. For example, the preform 100 may be prepared in a form ofpellet by cold pressing after blending or ball-milling using rawpowders.

The preform 100 is put into a crucible 120 in which an aluminum (oraluminum alloy) melt 110 is placed, and is immersed in the aluminum melt110. In the present specification and the claims, the aluminum melt 110all refers to a melt of pure aluminum, or a melt of an aluminum alloy towhich additional elements (Mg, Si, Cu, Mn, Cr, Zn, Ni, Ti, Fe, Sn, Li,and the like) of a typical aluminum alloy are added.

Here, part of the preform is immersed so as to be exposed to an externalenvironment, for example, the atmosphere without being immersed in thealuminum molten metal 110. Representatively, as in FIG. 1, the uppersurface of the preform 100 may not be immersed in the aluminum melt 110by exposing the upper surface of the preform 100 above the surface ofthe melt.

The preform 100 injected into the aluminum melt 110 receives heat fromthe aluminum melt 110, and a combustion synthesis reaction occurs withinseveral ten seconds and up to several minutes while the preform 100 isheated. Simultaneous with the combustion synthesis reaction, moltenaluminum is infiltrated into the preform 100 due to a pressuredifference between the inside and the outside of the preform 100. Whenthe preform 100 is completely infiltrated, an aluminum matrix compositeis prepared by taking the preform 100 out of the aluminum melt 110, andsolidifying the preform 100.

According to the present embodiment, the preform 100 having cavitiestherein forms a hard ceramic, such as carbide, oxide, and boride by acombustion synthesis reaction, and as molten aluminum is infiltratedthrough the cavities inside the preform 100, an aluminum matrixcomposite in which a hard ceramic is distributed in an aluminum matrixis prepared.

In the present embodiment, an important factor for inducing infiltrationof the molten metal into the preform 100 is the pressure difference, anda basic principle of generating the difference is due to the followingtwo factors.

(1) Generation of Pressure Difference Inside and Outside of Preform 100

A preform formed by a raw powder mixture usually has a density which is50 to 80% of a theoretical density. That is, 20 to 50% of the preform isoccupied inevitably by the air and gas. Further, moisture or gas, andthe like are adsorbed on the surface of the raw powders. When thepreform is in contact with the molten metal, or combustion synthesisreaction occurs inside of the preform, the internal temperature of thepreform is increased, and accordingly, the air, moisture, gas, and thelike present inside the preform are thermally activated. In conditionswhere the activated air, moisture, adsorbed gas and the like are easilyremoved from the preform, the inside of the preform may be temporarilyin a lower pressure condition which is close to the vacuum.

Furthermore, when the combustion synthesis reaction is occurred byreactions of the powder mixtures composing the preform, more cavitiescan be formed inside the preform due to volume contraction. In general,the volume of reacted powder is smaller than that of the raw powders.Due to this reason, an additional empty space is created inside thepreform. Accordingly, a pressure difference between the inside and theoutside of the preform is generated by the aforementioned two factors,and the aluminum melt is spontaneously infiltrated into the empty space.

In the related art, a pressure difference was generated artificially byusing a vacuum generation device. In comparison with the related art, inthe present invention, it is possible to make inside of the preformnearly vacuum state just simple contact with melt or reaction synthesisheat.

(2) Action of Capillary Pressure

When a rigid body having cavity therein is brought into contact with aliquid, the liquid is sucked into the inside of the rigid body by acapillary pressure, and accordingly, the rigid body may be infiltratedinto the liquid. In this case, the acting pressure may be expressed bythe following Equation (1).

$\begin{matrix}{P_{c} = \frac{2r_{1v}\cos \; \theta}{r_{c}}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, P_(c) is a capillary pressure, γ_(1v) is a surface tension of theliquid, θ is a contact angle between the liquid and the solid, and r_(c)is a radius of the capillary. FIG. 2 illustrates a schematic view inwhich the molten metal is infiltrated by capillary pressure. As shown inEquation 1, when the contact angle θ is larger than 90°, P_(c) has anegative value, and the liquid cannot be infiltrated into the inside therigid body by capillary pressure. That is, the molten metal cannot beinfiltrated into the preform spontaneously, and additional externalpressure is needed to infiltrate the molten metal into the preform. Onthe contrary, when the contact angle θ is smaller than 90°, P_(c) has apositive value, and the liquid can be infiltrated spontaneously into therigid body by to the capillary pressure. That is, the molten metal canbe spontaneously infiltrated into the preform.

In general, the contact angle between molten aluminum and ceramicparticle has a value larger than 90°. However, the contact angle is nota constant value, and is a value which varies according to time andtemperature. For example, when molten aluminum is in contact with B₄C,the contact angle is 100° in the case where the contact is maintained at900° for 1 second, but the contact angle is decreased to 90° in the casewhere the contact is maintained at the same temperature for 1 hour, andthe contact angle is further decreased to 60° or less in the case wherethe contact is maintained at 1,200° C. for 1 second (Q. Lin, ScriptaMat., 60, 2009, 960-963). That is, when the temperature of the liquid(or the molten metal) is increased, the contact angle may be decreasedto 90° or less, and the molten metal can be spontaneously infiltratedinto the preform due to the decrease in contact angle.

The present embodiment may easily produce nearly vacuum state andcapillary pressure using a combustion synthesis reaction, and mayprepare an aluminum matrix composite by spontaneously infiltrating thealuminum molten metal into the preform using the pressure difference.

Referring to FIG. 1 as described above, part of the preform 100 in thealuminum melt 110 is exposed to the external environment without beingimmersed in the aluminum melt 110, and the other parts thereof need tobe immersed in the aluminum melt 110 to be directly in contact with thealuminum melt 110. This can help the exit of the air, adsorbed gaspresent inside of the preform 100 to outside easily, and molten aluminumis more easily infiltrated into the preform 100 through a surfacecontacted with the aluminum melt 110. The infiltration step may beperformed at a pressure lower than the atmosphere or under a vacuumatmosphere as well as in the case where the external environment is theatmosphere.

In the step of infiltrating molten aluminum into the preform, thepreform may be ignited while the combustion synthesis reaction rapidlyproceeds. When the ignition occurs, molten aluminum is rapidlyinfiltrated by a combustion synthesis reaction.

The mixture of raw powders which compose the preform is not limited aslong as the mixture itself enables the combustion synthesis reaction,and a product produced in an aluminum alloy matrix after the combustionsynthesis reaction includes a product formed of at least one combinationof hard ceramics, such as carbide, oxide, and boride. Equations (2) to(8) show examples of the combustion synthesis reaction which may be usedin the present embodiment.

3Ti+B₄C→2TiB₂+TiC  (Equation 2)

4Al+2B₂O₃+C→B₄C+2Al₂O₃  (Equation 3)

10Al+3TiO₂+3B₂O₃→2TiB₂+5Al₂O₃  (Equation 4)

Ti+C→TiC  (Equation 5)

4Al+3TiO₂+3C→3TiC+2Al₂O₃  (Equation 6)

2Al+Ti+B₂O₃→TiB₂+Al₂O₃  (Equation 7)

4Al+3TiO₂+B₄C→2TiB₂+TiC+2Al₂O₃  (Equation 8)

The left side of the reaction equations of (Equation 2) to (Equation 8)indicates raw materials constituting the preform, and the right side ofthe reaction equations corresponds to the reinforcing phase of acomposite as a product produced by a combustion synthesis reaction.Table 1 is a calculated result for volume contraction due to thereaction.

TABLE 1 Equation Ratio of volume contraction (%) Equation 2 19.7Equation 3 26.7 Equation 4 33.0 Equation 5 23.5 Equation 6 21.7 Equation7 28.9 Equation 8 20.0

It can be seen that volume contraction occurs by approximately 19 to 33%due to the combustion synthesis reaction, and the aluminum melt may bespontaneously infiltrated into the empty spaces after the combustionsynthesis reaction.

Table 2 illustrates an adiabatic temperature due to heat generated bythe reaction, and it can be seen that the adiabatic temperature isincreased by heat of the exothermic reaction.

TABLE 2 Equation Adiabatic Temperature (K) Equation 2 3304 Equation 32323 Equation 4 2682 Equation 5 3441 Equation 6 2368 Equation 7 3054Equation 8 2522

In the present embodiment, the mixture of raw powders may furtherinclude an aluminum powder in an excessive amount in addition to astoichiometric amount required for a combustion synthesis reaction amongthe raw powders mixture. This is because the reactions of Equations (2)to (8) are reactions using aluminum as an intermediate, and as anexample, the reaction of Equation (5) is finally completed via anintermediate reaction as in Equation (9), and Al introduced as anintermediate is reduced in an amount which is the same as the amount ofthe initial stage.

(13/3)Al+Ti+C→Al₃Ti+(1/3)Al₄C₃→TiC+(13/3)Al  (Equation 9)

Accordingly, the combustion synthesis reaction may be more activelyinduced by adding an aluminum powder in an excessive amount to the leftside (that is, raw powder mixture) of (Equation 2) to (Equation 8). Theamount of excess aluminum powder may be in a range of 0.5 mol to 15 molaccording to the type of reaction.

When an aluminum powder is included in excessive in the mixture of rawpowders, the present invention may further include activating powderswhich can promote the exothermic reactions. For example, the reactionsof (Equation 2) to (Equation 8) may be further promoted in an aluminummelt by further adding activating powders, which have high reactivitywith aluminum, to the reactions of (Equation 2) to (Equation 8). Theactivating powders may be a metal oxide, and may include one or more of,for example, Cu oxide (CuO), Mn oxide (MnO), Fe oxide (FeO), and Nioxide (NiO). Table 3 shows a change in adiabatic temperature by thereaction of the oxide with aluminum.

TABLE 3 Equation Adiabatic Temperature (K) Cu oxide 3044 Ni oxide 3183Mn oxide 2474 Fe oxide 3133

As an example, the Cu oxide CuO is an oxide which exhibits a highexothermic reaction when reacted with aluminum, and the followingreaction occurs.

2Al+3CuO→Al₂O₃+3Cu  (Equation 10)

When the adiabatic temperature by the reaction of (Equation 10) iscalculated, it can be seen that the temperature may reach 3,044 K (seeTable 3), and the reactions of (Equation 2) to (Equation 8) may bepromoted by the addition of CuO.

As for the content of activating powder to be added, it is preferred toadd a content of 0.01 to 3 moles based on the mole content. The higherthe content of activating powder to be added is, the more rapid thereaction is, but when the activating powder is added in an extremelylarge amount, in the case of a metal component produced by decompositionof the activating powder, for example, CuO, Cu remains in the aluminummelt, and may unnecessarily increase the content of Al₂O₃.

In the present invention, the temperature of the aluminum melt may bemaintained in a range of 750° C. to 950° C. At less than 750° C.,infiltration by molten aluminum may rarely occur. In particular, whenthe mixture of raw powders includes an aluminum powder in an excessiveamount, the excess aluminum powder absorbs the reaction heat, therebydecreasing the adiabatic temperature. In this case, the combustionsynthesis reaction is prolonged to a longer time, or even no reactionmay occur. Accordingly, in consideration of this point, it is preferredto maintain the temperature of the aluminum melt at 750° C. or more.Meanwhile, the content of hydrogen gas in the aluminum melt isinevitably increased when the temperature of the aluminum melt isincreased, so possibility of presence of pores inside the infiltratedpreform is increased after the process is completed. Further, due to aneed for an additional device for increasing the temperature, productioncosts are increased, and the process becomes complicated. Accordingly,in the embodiment, it is preferred to perform the process at thetemperature of the aluminum melt of 950° C. or less, rigorously 920° C.or less.

Meanwhile, after the infiltrating of molten aluminum into the preform100 is completed while a part of the preform 100 is exposed to anexternal environment, the embodiment may further include stabilizing thepreform 100 by completely immersing the entire preform 100 in thealuminum melt 110 and maintaining the preform 100 in the aluminum melt110 for a predetermined time before finally taking the preform 100 outof the aluminum melt 110 and solidifying the preform 100. This may be afinal step for more securely infiltrating molten aluminum into thepreform 100.

In order to perform a series of processes more easily according to thepresent invention, it is possible to use a holder for holding thepreform 100 stably. As an example, as illustrated in FIGS. 9 and 10, itis possible to manufacture and use a ring type holder which supportsonly the border of the preform in order to facilitate the work. In thecase of the holder, the contact surface with the molten metal is so widethat it is easy to infiltrate the molten metal, and the holder is easilyapplied even to an arbitrary shape of preform and the upper portion ofthe holder may be filled with the molten metal. As another example, itis also possible to use a tube type holder in which only the upper andlower portions of the holder is open. When the holder is used, there isan advantage in that the shape of the preform may be more completelymaintained.

In addition, in the present invention, in order to induce a partialcombustion synthesis reaction, a non-metallic raw powder may be added inthe mixture of raw powders in an excessive amount equal to or more thana stoichiometric amount required for the combustion synthesis reaction.And, it is also possible to allow the reactant to remain by inducing apartial reaction in some cases.

For example, when the B₄C powder is used as a reactant as in Equations 2and 8, it is also possible to add B₄C in an excessive amount to reactthe B₄C partially (see the following Equations 11 and 12), and allow theexcessively added B₄C to remain in the preform.

3Ti+(1+x)B₄C→2TiB₂+TiC+xB₄C  (Equation 11)

4Al+3TiO₂+(1+x)B₄C→2TiB₂+TiC+2Al₂O₃+xB₄C  (Equation 12)

As another example, the mixture of raw powders may further include acompound powder which does not participate in the combustion synthesisreaction. For example, in (Equation 3), B₄C or SiC, TiC, Al₂O₃, or thelike is added in an excessive amount. The following Equations 13 and 14exemplify the case where B₄C and SiC are added to (Equation 3).

4Al+2B₂O₃+C+xB₄C→(1+x)B₄C+2Al₂O₃  (Equation 13)

4Al+2B₂O₃+C+SiC→SiC+B₄C+2Al₂O₃  (Equation 14)

A stable compound to be added in an excessive amount in order to inducea partial reaction as described above is not directly involved in thereaction and may maintain the shape of the preform more stably asillustrated in FIG. 2, and thus may serve to remove the air, moisture,adsorbed gas and the like in the preform more easily, and the compounditself is a reinforcing phase having excellent properties, and thusserves to enhance properties of a metal composite to be prepared.

Hereinafter, experimental examples will be provided in order to helpunderstand the present invention. However, the following experimentalexamples described below are only for helping to understand the presentinvention, and the present invention is not limited by the experimentalexamples below.

Table 4 shows the compositions of the raw powders which followExperimental Examples 1 and 2 of the present invention.

TABLE 4 Basically added component Excessively added component andcontent thereof (mole) and content thereof (mole) Result No. Ti B₄C AlCuO B₄C Infiltration Produced phase Experimental Example 1 3 1 6 0.4 3Successful TiB₂, B₄C Experimental Example 2 3 1 1 0.1 0 Successful TiB₂,B₄C, TiC

The raw powders with the compositions shown in Experimental Example 1were maintained at a temperature of 180° C. for 1 hour and dried, andthen mixed by using a ball-mill. A preform was prepared by compressingthe mixed powder using a press device, and the applied pressure wascontrolled during the preparation to allow a density of the preform tobe 60% of the theoretical density. The shape of preform was 35 mm indiameter and 28 mm in thickness, and several identical preforms wereprepared.

The prepared preforms were introduced into aluminum melt which wasmaintained at 900° C. in the atmosphere, and during the introduction,the surface of the preform was allowed to be exposed to the surface ofthe molten metal as in FIG. 1. After the preform was ignited in thealuminum melt, the preform was kept for 3 minutes by immersing thepreform in the aluminum melt completely, and then the preform was takenout of the aluminum melt, and solidified in the atmosphere.

For some of the preforms, the upper portion of the preform wasperforated, and then a thermocouple was mounted to directly measure achange in temperature of the preform and the aluminum melt. FIG. 3illustrates the results, and it can be seen that when the preform wasintroduced into the molten metal, the temperature of the preform wasgradually increased (heating stage), and when about 74 seconds elapsed,the temperature of the preform was sharply increased (about 1,156° C.)while the preform was ignited due to reaction in the preform (ignitionand infiltration stage). In the next stabilization stage, it can be seenthat the temperature of the preform becomes close to the temperature ofthe aluminum melt.

Some of the preforms were taken out of the aluminum melt, immediatelybefore the ignition (A of FIG. 3) and immediately after the ignition (Bof FIG. 3), and quenched in water to observe whether the aluminum wasinfiltrated thereinto. As a result of the observation, it can beconfirmed that aluminum melt was not infiltrated into the preform forthe preform taken out of immediately before ignition. But aluminum meltwas completely infiltrated into the preform for the preform taken out ofimmediately after ignition. That is, when the preform is ignited, it canbe confirmed that aluminum has been infiltrated into the preform in avery short time, and it can be confirmed that the entire process may beeasily completed in the atmosphere within 5 minutes. In order to verifywhether the test results as described above are theoretically feasible,the relationship between the infiltration length-infiltration time wastheoretically calculated based on the size of initial B₄C powder and thesize of final B₄C powder for the composition of the preform inExperimental Example 1. FIG. 4 is a view illustrating the result, andthe calculation result shows that the infiltration can be completedwithin 2 to 3 seconds.

FIGS. 5 a and 5 b illustrate a microstructure observed at lowmagnification and high magnification, respectively, for the preform inExperimental Example 1. It can be seen that the aluminum melt had beensuccessfully infiltrated, so that the preform had a sound structurehaving few pores therein. Since a partial reaction was used in thepresent experimental example, it can be observed that a large amount ofB₄C remained in the microstructure, and it can be also confirmed thatfine TiB2 phase (brown color) was formed around B₄C due to thereactions. The properties of the sample are summarized in Table 5. Thesample was a composite composed of Al-TiB₂—B₄C. The composite samplesexhibited lower density as 2.94 g/cc, excellent mechanical propertiessuch as an elastic modulus of 158 GPa and a hardness of 166 kg_(f)/mm²(1.63 GPa), and had a coefficient of thermal expansion (CTE) of 9.4ppm/K. Thus it is expected to utilize the composites as parts where highhardness, high stiffness and thermal stability are required.

TABLE 5 Item Measured value Density 2.94 g/cc Elastic coefficient 158GPa Hardness 166 kg_(f)/mm² (1.63 GPa) Thermal expansion 9.4 ppm/Kcoefficient

For the composition which followed Experimental Example 2, a preform wasprepared in the same manner as in Experimental Example 1. FIGS. 6 a and6 b illustrate a microstructure observed at low magnification and highmagnification, respectively, for the preform in Experimental Example 2.It can be seen that the aluminum melt had been successfully infiltrated,so that the preform had a sound structure having few pores therein.Since a complete reaction was used in the present experimental example,no B₄C was remained in the microstructure; whole B₄C was reacted to formreaction products and it can be seen that fine TiB₂ phase (brown color,approximately 1 μm in size) and TiC phase (gray color, approximately 1μm in size) had been successfully formed in the microstructure.

Table 6 shows the compositions of the raw powders which followExperimental Examples 5 and 6. A test, which is the same as inExperimental Example 2, was performed by using a raw powder with thecomposition shown in Table 6. FIGS. 7 a and 7 b illustrate amicrostructure observed at low magnification and high magnification,respectively, for Experimental Example 5. It can be seen that thealuminum melt had been successfully infiltrated, so that the preform hada sound structure having few pores therein, and it can be seen thatAl₂O₃ phase (dark gray) was present in a network form in themicrostructure by the reaction, and B₄C phase (light gray) having asquare shape had been produced along the border of the network.

TABLE 6 Basically added component Excessively added component andcontent thereof (mole) and content thereof (mole) Result No. Al B₂O₃ CAl CuO B₄C Infiltration Produced phase Experimental Example 5 4 2 1 31.5 0 Successful Al₂O₃, B₄C Experimental Example 6 4 2 1 5 1 1Successful Al₂O₃, B₄C

Table 7 shows the compositions of the raw powders which followExperimental Examples 8 and 9. A test, which is the same as inExperimental Example 2, was performed by using a raw powder with thecomposition shown in Table 7. FIGS. 8 a and 8 b illustrate amicrostructure observed at low magnification and high magnification,respectively, for Experimental Example 8. It can be seen that thealuminum melt had been successfully infiltrated, so that the preform hada sound structure having few pores therein, and it can be seen thatcoarse Al₂O₃ phase (dark gray) and a brown TiB₂ phase, which is as fineas 1 had been produced in the microstructure by the reaction.

TABLE 7 Basically added component Excessively added component andcontent thereof (mole) and content thereof (mole) Result No. Al TiO₂B₂O₃ Al CuO B₄C Infiltration Produced phase Experimental Example 8 10 33 8 1.5 0 Successful Al₂O₃, TiB₂, Experimental Example 9 10 3 3 8 1.5 1Successful Al₂O₃, TiB₂, B₄C

The present invention has been described with reference to theembodiments illustrated in the drawings, but the embodiments are onlyillustrative, and it would be appreciated by those skilled in the artthat various modifications and other equivalent embodiments can be made.Therefore, the true technical scope of the present invention shall bedefined by the technical spirit of the appended claims.

EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS

-   -   100: Preform    -   110: Aluminum melt    -   120: Crucible

1. A method for preparing an aluminum matrix composite usingpressureless infiltration, the method comprising: preparing a preformformed of a mixture of raw powders capable of forming ceramic through acombustion synthesis reaction; immersing the preform in an aluminummelt, in which a part of the preform is exposed to an externalenvironment without being immersed in the aluminum molten metal; andinfiltrating molten aluminum into the preform while causing a combustionsynthesis reaction in the preform.
 2. The method of claim 1, wherein theinfiltrating of the molten aluminum further comprises allowing thepreform to be ignited.
 3. The method of claim 1, further comprising:immersing the entire preform in the aluminum melt after the infiltratingof the molten aluminum.
 4. The method of claim 1, wherein the mixture ofraw powders is any one of a mixture of Ti and B₄C powders, a mixture ofAl, B₂O₃, and C, a mixture of Al, B₂O₃, and TiO₂, a mixture of Ti and C,a mixture of Al, TiO₂, and C, a mixture of Al, Ti, and B₂O₃, and amixture of Al, TiO₂, and B₄C.
 5. The method of claim 1, wherein themixture of raw powders further comprises an aluminum powder in anexcessive amount in addition to a stoichiometric amount required for acombustion synthesis reaction among the raw powders constituting themixture.
 6. The method of claim 5, wherein the mixture of raw powdersfurther comprises an activating powder capable of causing an exothermicreaction with aluminum.
 7. The method of claim 6, wherein the activatingpowder comprises one or more of a copper oxide, a manganese oxide, aniron oxide, and a nickel oxide.
 8. The method of claim 1, wherein inorder to induce a partial combustion synthesis reaction, a non-metallicraw powder among the raw powders constituting the mixture is added inthe mixture of raw powders in an excessive amount equal to or more thana stoichiometric amount required for the combustion synthesis reaction.9. The method of claim 1, wherein the mixture of raw powders furthercomprises a stable compound powder which does not participate in thecombustion synthesis reaction.
 10. The method of claim 9, wherein thestable compound comprises one or more of B₄C, SiC, TiC, and Al₂O₃. 11.The method of claim 1, wherein a temperature of the aluminum melt is ina range of 750° C. to 950° C.